This invention relates to a method and apparatus for determining onboard a floating vessel being used to drill a subaqueous wellhole, the flow rate of drilling fluid flowing out of the wellhole into a telescoping marine riser connecting between the wellhole and the vessel. This invention has particular application to the early detection of the intrusion of formation fluids into the wellhole or the loss of drilling fluid from the wellhole to the formation.
In drilling a well, particularly an oil or gas well, there exists the danger of drilling into an earch formation that contains high pressure fluids. When this occurs, the high pressure fluid from the formation intrudes into the well and displaces the drilling fluid (mud) up the well. If this occurrence is not controlled rather quickly, the drilling fluid may be substantially displaced and the high pressure fluid may flow freely up the well. This is termed a blowout. On the other hand, the well may be drilled into an earth formation which is very porous. In such a situation, there may be a tendency for the drilling fluid to flow freely from the well into the surrounding earth formation. This is termed lost circulation.
Blowout prevention is most effective when the commencement of an influx of high pressure fluid into the well can be quickly detected and controlled before an appreciable amount of the drilling fluid is displaced from the well. Loss of drilling fluid is kept to a minimum when the commencement of the loss can be quickly detected and the flow of the fluid controlled before an appreciable amount has passed from the well into the earth formation. It is known in the art to detect such an influx or loss of fluid by comparing the flow rate of the drilling fluid into the well and the flow rate of the fluid returning out of the well. A substantial increase in the rate of the returning fluid flow when there was no corresponding increase in the rate of the fluid flow into the well, is indicative of a blowout. A substantial decrease in the rate of the returning fluid flow when there was no corresponding decrease in the rate of the fluid flow into the well, is indicative of lost circulation.
In drilling offshore subaqueous wells from floating vessels, such as ships, barges or semisubmersibles, the floating vessel is usually connected to the subaqueous wellhole by a marine riser. To accomodate the heaving motion of the vessel, the marine riser is usually provided with a telescoping section or slip joint. A hollow drill string extends downwardly from the vessel through the marine riser and into the wellhole. A drill bit is connected to the lower end of the drill string. The drill string also is usually provided with a telescoping joint (often called a "bumper sub"). Drilling fluid generally is pumped from the vessel through the hollow drill string downwardly to the drill bit. The drilling fluid flows out into the well through ports in or adjacent to the drill bit and circulates back up to the vessel through the annulus between the drill string and the marine riser.
The heaving motion of the vessel strokes the telescoping joint in the marine riser causing it to extend and contract, thereby increasing and decreasing the volume of the flow path of the drilling fluid. This results in pulsations in the rate at which the returning drilling fluid is received from the marine riser onboard the vessel. The instantaneous maximum and minimum flow rate of the returning drilling fluid induced by the extension and contraction of the marine riser may be several times greater or less than the steady state or real flow rate. In most systems for drilling subaqueous wells from floating vessels, means are employed in the vicinity of the vessel for measuring the flow rate of the drilling fluid returning to the vessel from the marine riser. As such measurement is made above the telescoping marine riser, the cyclic variations in the volume of the marine riser caused by the movement of the vessel make it difficult to determine whether a substantial decrease or increase in the flow rate of the drilling fluid returning to the vessel is due to a blowout, lost circulation, or the extension and contraction of the riser. The real flow rate of the drilling fluid out of the wellhole into the telescoping marine riser is masked by the linear extension and contraction of the marine riser whereby it is difficult, if not impossible, to detect quickly the true flow rate of the returning drilling fluid.
Gadbois, in his U.S. Pat. No. 3,760,891, discloses a method and apparatus for rapidly detecting blowouts and lost circulation in a well, which method and apparatus has particular application in a well being drilled at sea from a heaving vessel. The Gadbois system monitors the return rate of flow of the drilling fluid in the vicinity of the vessel and generates an electrical signal proportional thereto. The electrical signal is monitored, accumulated, compared with selected samples of the accumulated signal, and compared with selected threshold values, to determine the existence of the blowout or lost circulation. The Gadbois system is very advantageous but does not provide a signal which is continuously and substantially instantaneously proportional to the true flow rate of the drilling fluid flowing out of the wellhole and into the annulus between the drill string and the marine riser. Gorusch, in his U.S. Pat. No. 3,602,322, discloses a system for determining an imbalance between the rates of flow of the drilling fluid into and out of a well. Gorsuch, however, does not disclose a system which can effectively deal with the oscillations in the rates of flow of the drilling fluid in a well being drilled at sea from a heaving vessel. Jefferies et al., in their U.S. application Ser. No. 508,883, filed Sept. 26, 1974, now U.S. Pat. No. 3,910,110 issued Oct. 7, 1975, disclose a system for detecting the commencement of a blowout or lost circulation in a subaqueous well in which the rate of flow of the drilling fluid flowing back to the vessel is measured, an electrical signal is generated proportional thereto, the electrical signal is modified to compensate for the change in the volume of the flow path caused by the heaving motion of the vessel, and the modified electrical signal is compared with another electrical signal proportional to the rate of flow of the drilling fluid into the well. Alternatively, the rates of flow of the drilling fluid into and out of the well are measured, compared, and a signal is generated proportional to the difference therebetween, and such electrical signal is modified to compensate for the change in volume of the flow path of the drilling fluid caused by the heaving motion of the vessel.
It is an object of this invention to provide an improved method and apparatus for determining onboard a floating vessel being utilized to drill a subaqueous wellhole, the true flow rate of drilling fluid flowing out of the wellhole into a telescoping marine riser connecting between the wellhole and the vessel.
It is a further object of this invention to provide an improved method and apparatus for measuring the flow rate of the drilling fluid being pumped from a floating vessel into a drill string which extends into a subaqueous wellhole, for measuring the flow rate of the drilling fluid flowing back to the vessel from a telescoping marine riser connecting between the wellhole and the vessel, and for rapidly determining the difference between the flow rate of the drilling fluid being pumped from the vessel into the drill string and the flow rate of the drilling fluid flowing out of the wellhole into the marine riser whereby the commencement of a blowout or lost circulation in the wellhole may be rapidly detected onboard the vessel.